1. Field of the Invention
The present invention relates to a sensor element and a gas sensor.
2. Description of the Related Art
There is so far known a gas sensor for detecting the concentration of a specific gas, e.g., NOx, in measurement object gas to be measured, such as automobile exhaust gas. For example, Patent Literatures (PTL) 1 and 2 disclose a gas sensor of the type including a sensor element that has an elongate plate-like shape, and that is formed by stacking a plurality of gas-tight solid electrolyte layers each having oxygen ion conductivity.
FIG. 14 is a schematic sectional view illustrating, in a simplified fashion, one example of a structure of a gas sensor 300 of the above-mentioned related art. As illustrated in the drawing, the gas sensor 300 includes a sensor element 307. The sensor element 307 is an element of a multilayer structure in which dense solid electrolyte layers 301 to 306 each having oxygen ion conductivity are stacked. In the sensor element 307, a measurement-object gas flowing portion through which measurement object gas is introduced is formed between a lower surface of the solid electrolyte layer 306 and an upper surface of the solid electrolyte layer 304. The measurement-object gas flowing portion includes a gas introducing region 310, and first to third inner cavities 320, 340 and 361. An inner pump electrode 322 is formed in the first inner cavity 320, an auxiliary pump electrode 351 is formed in the second inner cavity 340, and a measurement electrode 344 is formed in the third inner cavity 361. Furthermore, an outer pump electrode 323 is formed on an upper surface of the solid electrolyte layer 306. In the gas sensor 300, when the measurement object gas is introduced to the first inner cavity 320 in the measurement-object gas flowing portion, oxygen is pumped out or pumped in between the first inner cavity 320 and the outside of the sensor element 307 in accordance with a control voltage Vp0 that is applied between the outer pump electrode 323 and the inner pump electrode 322. Subsequently, when the measurement object gas is introduced to the second inner cavity 340, oxygen is pumped out or pumped in between the second inner cavity 340 and the outside of the sensor element 307 in accordance with a control voltage Vp1 that is applied between the outer pump electrode 323 and the auxiliary pump electrode 351. After the oxygen concentration of the measurement object gas has been controlled as described above during passage of the measurement object gas through the first inner cavity 320 and the second inner cavity 340, the measurement object gas is introduced to the third inner cavity 361. The concentration of a specific gas in the measurement object gas is then detected on the basis of a current Ip2 that flows when oxygen is pumped out or pumped in through the outer pump electrode 323 and the measurement electrode 344.